TOKYO — A strong Japan-South Korea military alliance is critical to any efforts to contain the situation on the Korean peninsula, the top U.S. military officer said Thursday.

Recent North Korean developments have helped strengthen ties among the U.S., Japan and South Korea. In a landmark meeting earlier this week, the three countries’ heads of state met in Washington to develop a firm and united message toward Pyongyang.

To further those efforts, Japan and South Korea should team up for direct military exercises, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said during a press conference at the U.S. Embassy in Tokyo following a meeting with his Japanese counterparts.

The inclusion of South Korean observers in the U.S.-Japan’s Keen Sword drill was “a terrific first step to broadening our trilateral relationship and deepening our collective readiness,” Mullen said.

Japan — which occupied Korea from 1910 until the end of World War II — also sent observers to a U.S.-South Korea exercise in July, although the two have yet to bring their militaries together for training.

Acknowledging the contentious history between the two countries, Mullen said: “The past I understand, but too often that drags us back when we should be moving forward.”

Japan’s empirical military rule of South Korea is “still a very sensitive issue” in South Korea, said Sheila Smith, senior fellow for Japan Studies at the Council on Foreign Relations in Washington. “Seoul’s comfort level must be respected” when forging new ties with Tokyo, particularly military interactions, she said.

Japan’s new government “has gone a long way toward reaching out to South Korea,” Smith said. “But it’s still a little early to think that South Korea would want the Japanese military working from inside the country.”

Echoing statements he made in Seoul the day before, Mullen called on China — North Korea’s top ally — to intervene in the current crisis on the Korean peninsula, blaming Beijng’s “inaction” thus far for enabling the North to continue its “abhorrent behavior.” Mullen arrived in Japan on Wednesday evening following his one-day stop in South Korea as the crisis on the Korean peninsula continues in the wake of the North’s shelling of Yeonpyeong island last month that killed four.

China’s influence on North Korea is particularly important following revelations by Pyongyang — a week after its Nov. 23 attack on the South — about its new advancement in developing nuclear weapons, Mullen said.

“The danger that presents to the peninsula, to the region and to the world, that really brings me and the U.S. government to a point where we’re saying this has got to stop,” he said. “Because if it doesn’t 10 years from now – whatever the right number – we’re going to be in a much more dangerous situation.”

While emphasizing the need for a more multilateral approach to regional defense in northeast Asia, Mullen stressed that “any actions and reactions have to be done very carefully to make sure, in fact, that we don’t escalate, that they are proportional and at the same time send a very strong signal that the provocations must cease.”

Mullen wouldn’t speculate when asked about how an all-out war on the Korean peninsula might play out.

“I don’t believe any country in the region – on either side – would want to see that happen,” he said. “It would just be calamitous.”

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